depending where you're from and the prices of GPUs there, you got a lot of good choices. if you're from the US or are ordering from it, there's the 7850, 7870, GTX 650 Ti Boost, & GTX 660. my vote's on the 660, or a 7870 if you wanna be picky about branding.
define "cheap". while you're at it; define your expectations. a GT 620 can still somewhat play games at the lowest settings + lowest res, disregarding good frame rate, at worst, .
depending where you're from and the prices of GPUs there, you got a lot of good choices. if you're from the US or are ordering from it, there's the 7850, 7870, GTX 650 Ti Boost, & GTX 660. my vote's on the 660, or a 7870 if you wanna be picky about branding.